The present invention relates to a matrix liquid crystal display device, more particularly, to a method of driving a matrix liquid crystal display device provided with switching transistors for respective picture elements.
Conventionally, it is well known that, even when a small-duty drive or a multi-line multiplex drive is performed, a high-contrast display equivalent to a static-drive display can be achieved in such a matrix liquid crystal display device using switching transistors built into the LCD panel. This is typically represented by an equivalent circuit shown in FIG. 1. In FIG. 1, reference number 11 indicates a switching transistor, which is conventionally composed of such a field effect transistor containing semiconductive elements such as a mono-crystal, multi-crystal or amorphous silicon (Si), tellurium (Te), or cadmium-selenium (CdSe) compound, etc. Reference number 12 indicates row electrodes and 13 column electrodes, which are respectively connected to the gate electrode and the source electrode of switching transistor 11. Reference number 14 indicates liquid-layer capacitors sandwiched between the display picture element electrode and the opposite electrode. Reference number 15 indicates charge storage capacitors, which are provided to compensate for insufficient charge capacitance of liquid-layer capacitors. In reference to the equivalent circuit shown in FIG. 1 and the drive signal waveforms in FIG. 2, functional principles of the liquid crystal display device are described below.
FIG. 2 (a) and (b) respectively show scan pulses applied to the first and (+1)th row electrodes 12. Such a pulse voltage containing the width H that turns the switching transistor 11 ON during the period H=T/N (where T denotes the total scan time and N the number of the scan line) is sequentially fed to each row electrode 12 so that each row electrode turns ON. FIG. 2 (c) indicates the data signal waveforms applied to the j-th column electrode 13. Transistors in each row sequentially turn ON at the j-th position. Voltage waveforms corresponding to such a voltage fed to the picture elements of respective rows are synchronously sent to the column electrode 13. FIG. 2 (c) indicates such a case in which V-volt is applied to the i-th picture element at the j-th position and 0-volt to all other picture elements. Data signal waveforms are fed in order to invert the polarity in each scan period to allow the liquid crystal display to be driven by AC power. With reference to FIG. 1, picture elements of the i-th row and the j-th column are described below. When the switching transistor 11 turns ON, the column electrodes start to charge the liquid crystal display's liquid capacitor 14 and the storage capacitor 15 via resistor RON of transistor 11, causing the potential of the display picture element electrodes to become the same +V as that of data signals. When the switching transistor 11 turns OFF, the charge remains unaffected allowing the +V potential of the display picture element electrodes to remain. When transistor 11 turns ON again, reverse charging is performed so that the potential of the display picture element electrodes becomes -V, which is maintained during the following OFF period. As a result, the display picture element electrodes will receive such a voltage waveform very close to the rectangular waveform shown in FIG. 2 (d). Liquid crystals are driven while the potential of the opposite electrodes is zero volt with the V-value above the threshold value of liquid crystals.
Next, picture elements in the (i+1)th row and the j-th column are described below. In this case, the display picture element electrode is charged to become zero volt and remains charged as shown in FIG. 2 (e), and as a result, no voltage is fed to liquid crystals, which then turn OFF. As described above, despite the multiplex drive thus performed, since such a stable voltage corresponding to the static driving can be supplied to liquid crystals, an extremely high-contrast display can be achieved using the liquid crystal display drive embodied by the present in- vention.
In the drive method described above, it is desirable that the time constants RON and CL (where RON denotes the onresistor of the switching transistors and CL the parallel capacitor of charge storage capacitors) be set substantially shorter than the scan pulse width H and also that charging of the capacitors sufficiently performed until the potential of the display picture element electrodes is equal to the voltage V of the data signal waveforms. This is because, if the time constants RON and CL are not less than the scan pulse width H and the display picture element electrodes are insufficiently charged, then even if the voltage V is fed to the column electrodes 13, liquid crystals cannot receive enough voltage. The liquid crystals receive voltage Vl only, as shown in FIG. 3. In addition, if such a condition exists, voltage Vl being fed to liquid crystals can be varied by the time constants RON and CL. As a result, if each picture element in the display unit contains varied values of the time constants RON and CL, such variations may adversely affect the display contrast effect, thus eventually causing a great obstruction against such a specific display requiring interim tones as in the display of TV pictures.
In the above driving method, the scan pulse width H can be calculated by H =(total scan time T).div.(number of scan lines N). However, since T cannot be set at such a large value due to possible flicker of liquid crystals, the value H cannot be set at any value greater than a specific limit. Since it is necessary to retain a specific charge, capacitance CL cannot be reduced. Likewise, on-resistor value cannot also be reduced when using a transistor containing semiconductive elements with low conductivity. These often cause such an obstruction mentioned above.